Broadband selective polarization antenna system



3 25 an H 3 4 SEARCH RGQEQE April 10, 1962 s. MILLER 3,029,431

BROADBAND SELECTIVE POLARIZATION ANTENNA SYSTEM Filed June so, 1960 2 Sheets-Sheet 1 TRANSCEIVER 2O INVENTOR L55. 5. MILLER ATTORZEY 5 L. S. MILLER A ril 10, 1962 BROADBAND SELECTIVE POLARIZATION ANTENNA SYSTEM Filed June 50, 1960 2 Sheets-Sheet 2 INVENTOR V LEE 5. M/LLER BY FIG.2.

ATTO N EY United States Patent 3,029,431 BROADBAND SELECTIVE POLARIZATION ANTENNA SYSTEM Lee S. Miller, Mountain View, Calif., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed June 30, 1960, Ser. No. 40,041 6 Claims. (Cl. 343-756) This invention relates to improvements in an antenna of the type used in radar systems, and more particularly to an antenna system for selectively radiating linearly and circularly polarized electromagnetic waves within a relatively wide range of frequencies.

Radar antennas generally are designed to use plane polarization. It is known, however, that under some weather conditions, such as heavy rain, there is considerable advantage in using circular polarization. This results from the fact that raindrops, which are spherical, reflect circularly polarized waves that are rotating in the opposite sense, whereas most targets are non-spherical and reflect but a portion of the circularly polarized transmitted waves, the reflected portion being elliptically or linearly polarized. Elliptically and linearly polarized waves contain both senses of polarization. It therefore is evident that an antenna system which radiates and receives circularly polarized waves of but one sense of circular polarization provides discrimination against the oppositely circularly polarized waves reflected from the raindrops, thus eliminating rain clutter from the radar system indicator presentation.

Under normal relatively clear weather conditions, it is found that the return signal from a particular target is usually about 7 db less when circular polarization is used than it is when plane polarization is used. Thus the radiation of circularly polarized waves is disadvantageous except during heavy rainfalls. It therefore is advantageous to provide an antenna system which selectively can radiate linearly polarized or circularly polarized electromagnetic waves, dependent upon the weather conditions.

In a marine radar system, for which the antenna of the present invention was designed, it is desirable that the radiated beam pattern be fan shaped, that is, narrow in azimuth coverage and relatively broad in elevation coverage. One means for obtaining a fan shape beam is by means of a cylindrical parabolic reflector whose aperture is broad in the azimuth plane and relatively narrow in the vertical plane, and wherein the reflector surface is a parabolic curve in the azimuth plane and uncurved in the vertical plane. Special consideration must be given when illuminating a reflector of this type with circularly polarized waves because the time and space quadrature relationship between the two equal-magnitude orthogonal components comprising the circularly polarized waves must be maintained over the entire surface of the reflector. In the past, pyramidal feed horns have been employed to illuminate the reflector with circularly polarized waves. This requires special structures in the born for independently adjusting the respective radiating apertures for the two orthogonal components in order to assure circular polarization of the energy over the entire reflecting surface. These horns, however, are useful only over a very narrow bandwidth because the respective propagating velocities of the two orthogonal components through the horn vary diflerently with frequency and cause the radiated energy to become elliptically polarized at operating frequencies diflerent from the particular design frequency of the horn.

It therefore is an object of this invention to provide an antenna structure capable of selectively radiating linearly or circularly polarized electromagnetic waves in a desired radiation pattern over a relatively wide range of frequencies.

Another object of this invention is to provide an antenna system for radiating circularly polarized electromagnetic waves over a relatively wide frequency range with a minimum variation in the circularity of the polarization.

A further object of this invention is to provide eflicient means for producing a fan shape radiation beam from a feed providing symmetrical and substantially equal radiation patterns for the respective orthogonal linear components of circularly polarized waves.

One of the important requirements considered in designing the antenna system of the present invention was that it should operate over a relatively wide frequency range in the circular polarization mode of operation. As previously noted, known horn radiators capable of radiating circularly polarized waves are quite limited in bandwidth. It was determined that a circular Waveguide feed of the type described by applicant in Electronics, March 1958, page 44, will operate in the circular polarization mode over at least a 12% bandwidth, and would be quite satisfactory to meet the bandwidth requirements. This type of circular feed horn produces symmetrical and substantially equal E plane and H plane radiation patterns for the respective orthogonal components of the circularly polarized waves; the respective patterns for the two orthogonal components being substantially equal. If this circular waveguide feed were to directly illuminate a cylindrical parabolic reflector in order to produce the desired fan shape beam there would be excessive spillover of the waves in the vertical plane of the antenna inasmuch as the vertical dimension of the reflector aperture must be smaller than the azimuth dimension in order to produce the fan shape beam. This, of course, is objectionable and is to be avoided. In order to avoid this objectionable feature and still be able to use the relatively wide bandwidth circular waveguide feed, I provide an antenna structure comprised of a cylindrical parabolic main reflector which is parabolic in the azimuth plane, and a sub reflector positioned in front of the main reflector for reflecting energy from the feed onto said main reflector. This subreflector has a concave parabolic curve in the vertical plane to produce the desired beam shape in the elevation plane (the main reflector produces no collimation in the elevation plane). Because the main reflector is considerably wider than it is high, and because of the symmetrical characteristics of the circular waveguide feed pattern, a sub reflector of cylindrical parabolic shape, for example, would not be able to provide complete azimuth illumination of the main reflector if the cylindrical parabolic sub reflector were positioned so that its apparent source was at the focus of the main reflector. In order to provide the necessary azimuth illumination of the main reflector, the sub reflector of the present invention has a convex cylindrical shape in the azimuth plane. This shaped surface increases the beam width of the feed pattern in the azimuth plane to completely illuminate the main reflector. The convex shape of the sub reflector permits a reduction of the focal length of the main reflector and thus reduces the overall antenna system dimension in the azimuth plane. Having chosen the broadband circular waveguide feed, and because of the necessity to avoid the cut off condition therein, it is not possible to reduce the size of the circular Waveguide feed to broaden the feed beam pattern in an attempt to obtain a shorter focal length for the main reflector.

In the present invention the circular waveguide feed is displaced below the sub reflector and is aligned to radiate upwardly. The sub reflector therefore is inclined at an angle to the central axis of the waveguide feed and reflects energy from the feed upwardly onto the main reflector, and because the sub reflector is positioned in front of the lower portion of the main reflector, aperture blocking of the main reflector by the sub reflector is substantially eliminated.

The present invention will be described by referring to the accompanying drawings wherein:

FIG. 1 is a perspective view of the antenna system of the present invention;

FIG. 2 is a schematic illustration used to help explain the operation of the antenna system in the elevation plane; and

FIG. 3 is a schematic illustration used to help explain the operation of the antenna system in the azimuth plane.

The antenna system illustrated in the perspective view of FIG. 1 includes a cylindrical parabolic main reflector 11 for producing a fan shaped radiated beam. Reflector 11 is parabolic across its broad, or horizontal, dimension to provide a relatively narrow azimuth coverage, and is linear along its narrow, or height, dimension and produces no collimation in the elevation plane. Facing main reflector 11 and disposed opposite the lower portion thereof is an inclined sub reflector 12 which has a convex circular shape across its broad, or horizontal, dimension and a concave parabolic shape across its narrow, or height, dimension. The bottom edge of sub reflector 12 is the vertex of the parabolic curve, although it need not be. In FIG. 1, the line extending between main reflector 11 and sub reflector 12 is a horizontal line and is intended to help illustrate the relative vertical positions of said reflectors.

A circular waveguide feed 13 radiates electromagnetic waves upwardly onto sub reflector 12, which in turn illuminates main reflector 11. Circular waveguide feed 13 is comprised of circular waveguide section 14 having a circular aperture 15 and a quarter-wave conductive flange 16 which extends perpendicularly about, and flush with, the aperture 15. This waveguide feed is of the type described by applicant in the above-referenced article. Circular waveguide section 14 is coupled to a section of circular waveguide 17 and is rotatable with respect thereto about its central axis by means of rotary joint 18. Disposed within waveguide section 17 is a quarter-wave plate 19 of a type well known in the art for producing circularly polarized waves from incident linearly polarized waves. Circular waveguide section 14 is rotatable through an angle of 45 so that in one extreme position the quarterwave plate will have substantially no effect on the instant linearly polarized waves, and linearly polarized waves will be radiated from circular aperture 15. In the system here described, the radiated linearly polarized waves are horizontally polarized. Upon rotating circular waveguide section 17 through an angle of 45 to its opposite extreme position, the quarter-wave plate 19 will be disposed at an angle of 45 to the electric field lines of the waves and will delay one of the orthogonal components of the linear polarized wave by 90 electric degrees with respect to the other orthogonal component, thus producing circularly polarized Waves which will radiate from circular aperture 15. A sheet of low loss dielectric material may be placed over the circular waveguide feed 13 to provide weather protection. Rotatable waveguide section 14 is coupled through rotary joint 18 and waveguide section 17 to a typical radar transmitter-receiver combination 20.

The quarter-wave flange 16 about circular aperture 15 has been found to produce substantially symmetrical and equal radiation patterns over a relatively wide frequency range for the two orthogonal electric field polarizations of the circularly polarized waves. Circular waveguide feed 13 produces a radiation pattern having a half-power beamwidth of approximately 60.

An antenna system constructed according to the present invention and intended to operate in the X band of microwave frequencies at a center frequency of 9375 mc., had the following approximate physical characteristics:

Diameter of feed aperture 15 inches 1.25 Height of sub reflector 12 "do-.." 7.55 Width of sub reflector 12 do 10.5 Equation of parabolic curve of sub reflector 1'2 y =5x Radius of curvature of sub reflector 12 in azimuth plane:

At center of surface inches 14.0 At top of surface do 14.452 At bottom of surface do 15.427 Width of main reflector 11 feet 7.0 Height of main reflector 11 ..do 2.5

The manner in which the antenna arrangement of the present invention operates to produce a fan shaped far field radiation pattern which is relatively broad in the elevation plane and relatively narrow in the azimuth plane may be understood by referring to FIGS. 2 and 3. In FIG. 2, a cross-sectional view taken in the elevation plane of FIG. 1, circular waveguide section 14 is aligned to radiate upwardly, and in the example here described, circular waveguide feed 13 produces a symmetrical radiation pattern approximately 60 wide at the half-power points. Sub reflector 12 has the shape of a portion of a parabola in the elevation plane, the bottom edge of reflector 12 being approximately at the vertex of the parabolic curve and the top edge of the sub reflector extending sufliciently over circular feed 13 so that the reflector intercepts the feed radiation pat-tern at approximately the one-tenth power points at both edges. The portion of the parabolic curve of sub reflector 13 is tilted, or inclined so that the axis of the parabola makes an angle of approximately 19 clockwise from the plane of the circular feed aperture 15. Sub reflector 12 functions to narrow the feed radiation beam in the elevation plane to an angle of approximately 15 at the half-power points. Main reflector 11 is not curved in the elevation plane and functions merely as a plane mirror to the sub reflector pattern, thus producing a far field radiation pattern having an elevation beamwidth of substantially 15 Main reflector 11 is inclined toward sub reflector 12 so that the central axis of the radiated beam is substantially horizontal, as is required in a marine radar system.

Because of the arrangement of upwardly-radiating circular feed 13 relative to sub reflector 12, and the arrangement of sub reflector 12 relative to main reflector 11, aperture blocking of both reflectors is substantially eliminated.

The focussing action of the antenna system in the azimuth plane is illustrated in FIG. 3. Sub reflector 12 has a convex cylindrical shape in the azimuth plane and causes the radiation pattern from circular waveguide feed 13 to diverge in the azimuth plane. This diverging beam illuminates the entire main reflector 11 Whose shape is a parabolic curve with a relatively wide (7 foot) aperture dimension in the azimuth plane, so that upon reflection from main reflector 11 the waves are collimated into a beam with a very narrow azimuth coverage (approximately 1.2 at the half-power points). I provide the convex cylindrical shape in the azimuth plane of sub reflector 12 inasmuch as a cylindrical parabolic reflector positioned so that its apparent line source coincided with the focus of main reflector 11 would be unable to provide complete azimuth illumination of main reflector 11 when illuminated by the given symmetrical pattern of broadband circular feed 13.

Several advantageous features result from providing a sub reflector having a cylindrical convex shape in the azimuth plane. The first being the above-mentioned feature that with the given symmetrical beam pattern from circular feed 13 the entire azimuth dimension of main reflector 11 is properly illuminated to produce a fan shaped beam, and secondly, the apparent line source of the cylindrical convex sub reflector 12 now is closer to the sub reflector than it wouldbe if the curve were a cylindrical parabolic curve. Thus, when the apparent line source of the cylindrical convex sub reflector 12 is positioned to coincide with the focus of cylindrical parabolic main reflector 11, sub reflector 12 will be positioned relatively farther from main reflector 11 and relatively closer to the focal point of reflector. This avoids aperture blocking of the main reflector 11 by sub reflector 12. Also, the convex shape of sub reflector 12 in the azimuth plane shortens the required focal length of the main antenna 11 and reduces the overall extent of the antenna system in the azimuth plane. Additional advantageous features are that the use of the sub reflector permits waveguide feed 13 to be positioned closer to the main reflector, thus making the antenna structure more compact, while at the same time avoiding aperture blocking of the sub reflector.

In the azimuth focussing of the antenna system the effective source appears as a line source, designated ALS, positioned immediately behind sub reflector 12, as illustrated in both FIGS. 2 and 3.

It therefore may be seen that the antenna system of the present invention is capable of radiating both circularly and linearly polarized waves in a fan shape radiation pattern over a relatively broad frequency range, this being accomplished by employing a circular waveguide feed having a quarter-wavelength flange at the aperture so as to provide substantially symmetrical and equal radiation patterns over a wide frequency range for the two orthogonal components of the circularly polarized waves, and by providing two reflecting surfaces wherein the first, or sub reflector produces the desired rather broad elevation beam pattern, and the second, or main, reflector produces the desired narrow beamwidth in the azimuth plane.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A broadband antenna system for selectively radiating linearly or circularly polarized electromagnetic Waves in a fan shaped beam comprising antenna feedmeans for selectively radiating over a relatively broad range of microwave frequencies linearly polarized or circularly polarized electromagnetic waves in a radiation pattern substantially symmetrical about the beam axis, said feed means comprising an upwardly-radiating open-ended circular waveguide having a conductive quarter-wave flange disposed perpendicular to and flush with the open end of said waveguide, a sub reflector having a concave parabolic shape in the elevation plane and a cylindrical convex shape in the azimuth plane for narrowing the feed radiation pattern in the elevation plane and widening the feed radiation pattern in the azimuth plane, said feed means being positioned at the focus of the parabolic curve of said sub reflector, and a cylindrical parabolic main reflector positioned on the side of said feed means opposite the sub reflector to be illuminated by said sub reflector, said main reflector having a parabolic curve in the azimuth plane to produce a fan shaped beam having a narrow azimuth angle coverage.

2. The combination as claimed in claim 1 wherein the sub reflector is disposed opposite the lower portion of the main reflector to reflect electromagnetic waves from said upwardly radiating feed means across and upwardly onto said main reflector at a proper angle to avoid aperture blocking of said main reflector by said sub reflector.

3. The combination as claimed in claim 2 wherein the reflecting surface of said sub reflector is inclined at an oblique angle to the central axis of said waveguide and said main reflector is inclined toward said sub reflector.

4. A broadband antenna system for selectively radiating electromagnetic waves in a fan shaped radiation pattern comprising a first reflecting surface having a concave parabolic curve in one cross-sectional plane and a convex cylindrical curve in an orthogonal cross-sectional plane, an electromagnetic wave radiating means disposed at the focus of said parabolic curve for illuminating said reflector with electromagnetic waves in a directive radiation beam pattern substantially symmetrical about the beam axis, said first reflector narrowing said radiation beam in said one plane and broadening said radiation beam in said orthogonal plane, and a second larger reflecting surface having a parabolic curve in said orthogonal plane and being linear in said one plane, said first reflector being positioned with its apparent source in said orthogonal plane at the focus of said second reflector, said second reflector having an aperture dimension in said orthogonal plane greater than the aperture dimension of said first reflector in said first plane whereby said second reflector narrows the far field radiation beam of said waves in said orthogonal plane more than said first reflector narrows said radiation beam in said first plane, thereby to produce a fan shaped radiation beam.

5. A directive antenna system for producing a fan shaped radiation pattern comprising antenna feed means for selectively producing linearly or circularly polarized electromagnetic waves in a directive radiation pattern substantially symmetrical about the beam axis, a sub reflector positioned in front of and inclined with respect to the aperture of said feed means for narrowing the feed radiation pattern in the elevation plane and widening the feed radiation pattern in the azimuth plane, the surface of said sub reflector comprising in the elevation plane a portion of a concave parabolic curve whose edge nearest said radiator includes, as a limit, the vertex of said curve and in the azimuth plane comprising a convex cylindrical curve, and a cylindrical parabolic main reflector positioned above said sub reflector and on the side of said feed means opposite said sub reflector for imaging the sub reflector pattern in the elevation plane and for collimating the sub reflector pattern into a narrow beam in the azimuth plane.

6. A broadband antenna system for selectively radiating linearly or circularly polarized electromagnetic waves in a fan shaped beam comprising antenna feed means for selectively radiating over a relatively broad range of microwave frequencies linearly polarized or circularly polarized electromagnetic waves in a radiation pattern substantially symmetrical about the beam axis, said feed means comprising an upwardly-radiating open-ended circular waveguide having a conductive quarter-wave flange disposed perpendicular to and flush with the open end of said waveguide, a sub reflector having a concave parabolic shape in the elevation plane and a cylindrical convex shape in the azimuth plane for narrowing the feed radiation pattern in the elevation plane and widening the feed radiation pattern in the azimuth plane, said feed means being positioned at the focus of the parabolic curve of said sub reflector, the parabolic curve of said sub reflector being a portion of a parabola whose edge nearest said feed means includes, as a limit, the vertex of the parabola, whereby aperture blocking of said sub reflector by said feed means is avoided, and a cylindrical parabolic main reflector positioned on the side of said feed means opposite the sub reflector to be illuminated by said sub reflector, said main reflector having a parabolic curve in the azimuth plane to produce a fan shaped beam having a narrow azimuth angle coverage.

References Cited in the file of this patent UNITED STATES PATENTS 

